Data storage medium with laterally magnetized pad and method for making same

ABSTRACT

An information storage medium with an array of laterally magnetized dots, as well as a process for producing this medium is disclosed. Each dot ( 2 ) contains at least one magnetic domain formed by a thin layer ( 4 ) of at least a magnetic material laterally covering this flat material and deposited at oblique incidence relative to the normal (z) to the plane ( 6 ) of the array. The invention applies in particular to computer hard drives.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority based on International PatentApplication No. PCT/FR02/02299, entitled “Information Storage Mediumwith Laterally Magnetised Dot Array, and Process for Producing SaidMedium” by Bernard Rodmacq, Stephane Landis, and Bernard Dieny, whichclaims priority of French application no. 01 08869, filed on Jul. 4,2001, and which was not published in English.

TECHNICAL FIELD

The present invention concerns an information storage medium, as well asa process for producing this medium.

It applies especially to hard computer drives.

PRIOR ART

An information storage medium, or memory, is currently constituted by athin and continuous layer of ferromagnetic grains. The magnetisation ofthese grains is directed in the plane of the layer and each bit ofinformation is made up of several grains whereof all magnetisation isparallel to the same direction.

If this direction is defined as corresponding to a zero, oppositemagnetisation will be defined as corresponding to a ‘one’ in binarynotation. A read/write head, while hovering above the layer offerromagnetic grains, can thus code information by locally creating amagnetic field for orienting magnetisation of each <<bit>> in onedirection or the other.

The density of the information stored on such a medium is limited by thesize of the <<bits>> and by the transition zones. In order to increasesthis density, various solutions have already been proposed:

utilising a continuous magnetic material whereof the magnetisation isperpendicular to the plane of the layer,

utilising a ‘discrete’ medium, that is, an array of magnetic dotsindependent of one another, each dot being monodomain, that is,corresponding to a single magnetic domain, the latter itselfcorresponding to an information bit. In this respect, reference is madeto the following document:

[1] S. Y. Chou, P. R. Krauss, L. Kong, ‘Nanolithographically definedmagnetic structures and quantum magnetic disk’, Journal of AppliedPhysics 79, 6101 (1996).

In the event where magnetisation of the magnetic material is parallel tothe plane of the array of dots (planar magnetisation), each dot can bemade monodomain by adjusting its shape (ellipse, for example), so as tofavour only two directions of magnetisation. But this type of array ismore complicated to manufacture than an array of dots in a square orrectangular shape.

In the event where magnetisation of the magnetic material isperpendicular to the plane of the array, the monodomain state is easierto obtain, and dots having a square or circular section can be used.

Nevertheless, the signal detected by the reading head is in principleweaker, as such materials with perpendicular magnetisation are generallyalloys or multilayers based on a magnetic element and a non-magneticelement, which reduces the field created in the vicinity of the dots.

Furthermore, in the case of a multilayer, it is known that the number ofrepetitions cannot be increased indefinitely (in order to increase theradiated field) since the magnetisation tends to be orientedprogressively in the plane under the effect of the demagnetising field.

DESCRIPTION OF THE INVENTION

The present invention proposes an information storage medium, comprisingan array of magnetic dots, said medium exhibiting good performance bothfor reading and for writing information.

Precisely, the object of the present invention is an information storagemedium, said medium being characterised in that it comprises an array ofdots of submicronic dimensions, said array being formed on a substrate,each dot containing at least one magnetic domain which has a directionof magnetisation and corresponds to a bit defined by this direction ofmagnetisation, said magnetic domain being formed by a thin layer of atleast one magnetic material laterally covering said dot.

According to a preferred embodiment of the medium forming the object ofthe invention, the direction of magnetisation is perpendicular to theplane of the array of dots.

The invention also concerns a process for producing the informationstorage medium which is the object of the invention, wherein themagnetic material is deposited onto the dots by sending to the latter afirst flux of atoms of this material at an oblique incidence relative tothe normal to the plane of the array, each dot shadowing the dot whichfollows it in the direction of the first flux, and the magnetic signalresulting from the magnetic material deposited on top of the dots and atthe base of the trenches separating rows of dots is neutralised, so thatonly a magnetic signal generated by the material deposited on the flankof the dots subsists.

The magnetic material may be selected from the group comprising iron,nickel, cobalt, alloys of the latter and magnetic materials havingstrong magnetisation, or one of the materials of this group, to which isadded one or a plurality of other elements, in variable quantity, forexample selected from chromium, tantalum, platinum, molybdenum andterbium, these elements enabling the magnetic properties of the thinlayer to be adjusted, such as the coercivity, the saturationmagnetisation and the magnetic anisotropy of this thin layer.

According to a particular embodiment of the process forming the objectof the invention, a resin covers the top of the dots before the magneticmaterial is deposited and this resin is eliminated after this magneticmaterial has been deposited, to thus obtain a first deposit on the sidesof the dots.

In this case, a second flux of atoms of a neutralisation material can besent to the array of dots at the same time as the first flux is sent andat a normal incidence to the plane of the array or at an obliqueincidence relative to the normal to the plane of the array, with anangle of incidence opposite that of the first flux relative to thisnormal, with the intensity of the second flux and the neutralisationmaterial being selected so that the alloy thus formed at the base of thetrenches with the magnetic material has magnetic properties different tothose of the first deposit obtained on the sides of the dots.

In the present invention, the first flux and at least a second flux ofatoms of a magnetic material can be sent to the dots, at obliqueincidences relative to the normal to the plane of the array, to formmagnetic deposits on a plurality of sides of each dot.

According to another particular embodiment of the process forming theobject of the invention, the magnetic material deposited on top of thedots and at the base of the trenches is eliminated, this eliminationresulting for example from ionic abrasion at an oblique incidencerelative to the normal to the plane of the array, with an angle ofincidence opposite that of the first flux.

According to another particular embodiment of the process forming theobject of the invention, a second flux of atoms is sent, at the sametime as the first flux, at an oblique incidence relative to the plane ofthe array, with an angle of incidence opposite that of the first flux,with the intensity and the nature of the atoms of the second flux beingcapable of resulting, along with the first flux, in the formation of anon-magnetic alloy on top of the dots and at the base of the trenches.

In this case, after the atoms of the first and second fluxes have beendeposited,

these first and second fluxes are inverted and the fluxes thus invertedare sent to the dots, or

the array is turned 180° around the normal to the plane of the array andthe first and second fluxes are again sent,

in order to obtain deposit of the atoms of the first and second fluxeson two opposite sides of each dot.

In the present invention, it is possible

to form the thin layer in the presence of an external magnetic field, or

after forming the thin layer, to thermally treat this thin layer in thepresence of an external magnetic field, or

to make a deposit of a layer of an antiferromagnetic material above orbelow the thin layer,

so as to induce an axis of magnetic anisotropy in the plane of this thinlayer.

In the invention, the dots can have a transverse, circular or ellipticalsection parallel to the plane of the array, the thin layer formedlaterally on each dot thus having a lateral thickness gradient, and thisgradient is used to induce an axis of magnetic anisotropy perpendicularto the plane of the array.

In the invention, a protective layer can also be formed on the entiresurface of the array, then this surface provided with the protectivelayer can be planarized to eliminate any relief.

Certainly, the technique of depositing material at oblique incidence isknown, but has never been used for producing structures of the type ofthose, which form the object of the present invention.

In the field of magnetic structures, this deposit at oblique incidencehas been used only for depositing a magnetic material onto the side of asingle line of etched silicon. In this respect, reference is made to thefollowing document:

[2] K. Matsuyama, S. Komatsu, Y. Nozaki, ‘Magnetic properties ofnanostructures wires deposited on the side edge of patterned thin film’,Journal of Applied Physics 87, 4724 (2000).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thedescription of the embodiments given hereinbelow, purely indicativelyand in no way limiting, with reference to the attached drawings, inwhich:

FIG. 1 is a schematic and partial perspective view of a particularembodiment of the magnetic information storage medium, forming theobject of the invention,

FIGS. 2A and 2B schematically illustrate processes for producing thismagnetic medium,

FIG. 3 is a schematic plan view of a dot of circular section formingpart of a magnetic medium according to the present invention,

FIG. 4 is a schematic illustration of different cases considered forcalculation of the component, following an axis normal to the plane ofan array of dots of a medium according to the present invention, of theexisting magnetic field above this array, and

FIG. 5 depicts the variations of this component as a function of thedistance to the centre of a dot, in these different cases in question.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

FIG. 1 is a schematic and partial perspective view of a particularembodiment of a magnetic information storage memory, which is composedfrom an array of submicronic dots 2.

These dots 2 are for example made of silicon, glass, aluminium orhardened polymer resin. The dimensions of the dots 2 as well as thespaces between adjacent dots are in a range from 10 nm to 500 nm.

In this example the array of dots comprises lines and columns ofparallelepipedic dots, the space between two adjacent lines isreferenced as e1, the space between two adjacent columns is referencedas e2, each dot at a height h, a depth d1, counted parallel to thecolumns, and a width d2, counted parallel to the lines, and each ofthese dimensions e1, e2, h, d1 and d2 belongs to the abovementionedrange. If dots having a transverse circular section (respectivelyelliptical) were to be used, the diameter (respectively the large axisand the small axis) of these dots would belong to this range.

Deposited on each of the dots 2 is a single thin layer, made from aclassic magnetic material (for example a ferromagnetic material), atoblique incidence.

An aim of the present invention is to effect magnetic deposit solely onone side or a plurality of sides of the dots. In the example in FIG. 1,this magnetic deposit 4 is formed on a single side of the dots 2.

A frame (x, y, z) is defined, wherein each of the axes x, y, z isperpendicular to the two others. The plane (x, y) is the plane of thearray, that is, the plane of the surface 6 of the substrate 8 from whichthe dots 2 are formed. The lines of the array are parallel to the axis yand its columns, to the axis x.

The axis z is perpendicular to this plane (x, y) and the plane (x, z) isthe plane of incidence of the flux of material from which the deposit 4is formed on each dot, that is, the plane to which this flux isparallel.

The result is that the magnetisation of the deposit 4 formed on the sideof each of the dots 2 is parallel to the plane (y, z) in the case of amagnetic material of planar magnetisation. This magnetisation,referenced as M in FIG. 1, can be directed along the axis y or along theaxis z.

In addition, if the axis of magnetic anisotropy can be orientedaccording to the axis z, constantly parallel to the plane (y, z), theresult is the equivalent of a material with perpendicular magnetisation.

However, with such orientation, an information storage medium accordingto the present invention has a higher magnetic signal (since themagnetic material utilised is pure and its total thickness is notlimited) and more spatially localised than an information storage mediumutilising a material with perpendicular magnetisation.

Such a medium according to the present invention also has greaterdeposit simplicity of the magnetic layer, as well as superior thermalstability, which allows total compatibility with conventional technicalproduction processes of information storage media.

A process for producing an information storage medium according to thepresent invention will be explained hereinbelow.

1) An array of dots of square, rectangular, circular or ellipticalsection is used, said array being obtained by a classic nanolithographyprocess or by the technique of nano-imprint from a substrate. Thesubstrate is for example made of silicon, glass, aluminium or hardenedpolymer resin.

2) To form the magnetic deposits, a classic technique for depositingthin metallic layers is used, for example cathodic sputtering orevaporation under vacuum.

3) The angle of incidence of the flux 10 (FIGS. 2A and 2B) of atoms ofthe magnetic material relative to the normal (axis z) to the plane (x,y) of the array of dots 2 is around 45°, and this flux 10 is parallel tothe plane (x, z) which forms the plane of incidence. In addition, thisflux is collimated (by means not shown here) so as to be the leastdivergent possible.

4) The angle of incidence, the height h of the dots 2 and the distancee1 between dots are selected in such a way that each dot forms a screenbetween the incident flux 10 and the following dot (in the direction offlux), so that there is or is not, according to the desired result,continuity between the deposit formed on the side of each dot and thedeposit formed at the base of the trench 12 separating this dot from thefollowing dot (this trench 12 being parallel to the axis y andseparating two lines of dots). Three of the four sides of each dot aswell as the part of the trench situated behind each dot (observed in thedirection of the flux 10) are thus protected and do not receive anydeposit.

In the case of FIG. 2A, the angle of incidence α is sufficiently small,considering the distance between dots and the height thereof, so thatthere is continuity between the deposit 14 formed on the side of a dotand the deposit 16 formed at the base of the adjacent trench 12.

In the case of FIG. 2B, the angle of incidence, designated β, is greaterand leads to non-continuity of the two deposits 14 and 16, the depositnot appearing on FIG. 2B because of the shadowing provided by theadjacent dot. It should be noted that there is always a deposit in thetrenches which are parallel to the axis x and thus separate the columnsof dots from one another.

5) The flux incident 10 is composed of atoms of a magnetic material(mainly iron, cobalt, nickel or alloys of the latter), and possiblycontaining other elements introduced in variable quantity (for examplechromium, tantalum, platinum, molybdenum and terbium) for the purpose ofadjusting the magnetic properties of the deposit.

6) Three different deposits are thus obtained:

a magnetic deposit 18 on the top of the dots 2 (in any form, for examplesubstantially square or rectangular parallel to the plane (x, y), orsubstantially round or elliptical, as is also indicated hereinbelow),

a magnetic deposit 14 on one of the sides of the dot (of square orrectangular form parallel to the plane (y, z)), and

a magnetic deposit, not shown, in the trenches parallel to the plane (x,z).

7) The magnetic signal emanating from the top of the dots and from thebase of the trenches parallel to the axis x is neutralised by one of themeans (A), (B) and (C) which are detailed hereinbelow.

(A) If the array of dots is formed by a classic nano-lithography process(comprising insolation of a photosensitive resin, then selective ionicabrasion), on completion of the production process for the dots, thefilm of photosensitive resin 20 is preserved, having served as mask onthe top of the dots. After deposit of the magnetic material, a solventenables the photosensitive resin to be removed and the magnetic deposit18 formed on the top of the dots.

The deposit of magnetic material in the base of the trenches which areparallel to the axis x is not inconvenient, since it gives out a muchweaker magnetic signal, given its distance from the apex of the dots. Inaddition, this deposit has the form of continuous parallel lines (thatis, very long compared to the size of the dots) and thus radiates a muchweaker magnetic field than that of the deposit formed on the side of thedots.

This deposit formed at the base of the trenches is also advantageouslyutilisable for canalising the lines of magnetic field emanating from agiven dot, which enables the state of magnetisation of the neighbouringdots not to be disturbed during the writing process.

If it is wished that this deposit formed at the base of the trenches tohave a coercivity different from that of the deposit formed on the sideof the dots, another material can be deposited at the same time as themagnetic material previously mentioned, at a normal incidence (flux 22in FIGS. 2A and 2B) or at an oblique incidence (flux 24 in FIGS. 2A and2B), though opposed to that of the flux 10: in FIGS. 2A and 2B, the flux24 is parallel to the plane (x, z) but makes an angle −α (FIG. 2A) or −β(FIG. 2B) with the axis z.

According to the nature and the rate of deposit of this other material,the coercivity of the alloy deposited at the base of the trenches can bevaried, said alloy resulting from the simultaneous deposit of the twomaterials.

(B) After having formed the deposit of magnetic material (flux 10 ofFIGS. 2A and 2B), ionic abrasion is carried out at an oblique incidenceand opposite to that which is utilised to form this deposit, with thisionic abrasion accordingly taking place along arrow 24 of FIGS. 2A and2B.

This allows the magnetic material deposited on the top of the dots andat the base of the trenches parallel to the axis x to be removed withoutdisturbing the deposit formed on the side of the dots.

(C) At the same time as the magnetic material is deposited, anon-magnetic material, for example chromium, is deposited at a normalincidence to the plane of the array of dots (arrow 22 of FIGS. 2A and23) or at an oblique incidence and opposite to that of the flux 10 andthus following arrow 24 of FIGS. 2A and 2B. The magnetic material thusforms an alloy with the non-magnetic material on the top of the dots andat the base of the trenches parallel to the axis x, the amount ofmaterial deposited on the sides of the dots being negligible in the caseof normal incidence to the plane of the array of dots.

The rate of deposit and the nature of the non-magnetic material areselected such that the formed alloy is itself non-magnetic.

It is also possible, (1) using the processes (A) and (C), to carry out aplurality of successive deposits and to turn the array of dots by 90° or180° in its plane between two successive deposits.

By way of example, in case (A), a first deposit of magnetic material canbe made, then the array can be turned, then a second deposit of theother material can be made and, in case (C), a first deposit of magneticmaterial can be made, then the array can be turned, then a seconddeposit of non-magnetic material can be made. The array can then beturned again, then the first deposit can be made, then the array can beturned, then the second deposit can be made and so on.

It is also possible, (2) using the process (A), to use a plurality offluxes of atoms of magnetic materials at oblique incidence.

In cases (1) and (2) above, magnetic deposits are made on several sidesof the dot, which increases the information density.

This is schematically illustrated in a plan view in FIG. 4 in which adot 2 is seen, a side of which is provided with the magnetic deposit 4and the other sides of which are respectively provided with magneticdeposits 26, 28 and 30.

8) An easy axis of magnetic anisotropy is induced in the verticaldeposit 14 on the side of each of the dots, this axis capable of beingdirected either parallel to the plane of the array of dots, that is,along the axis y, or perpendicularly to this plane that is, along theaxis z.

This can be realised either by forming the deposit of magnetic materialon the dots in the presence of an external magnetic field oriented alongone of the axes y and z, or by carrying out thermal treatment after thisdeposit in the presence of such a magnetic field.

If dots of transversal, circular or elliptical section (sectionperpendicular to the axis z) are used, perpendicular magneticanisotropy, that is, along the axis z, is favoured by the lateralgradient of thickness resulting from the used deposit geometry.

FIG. 3 is a schematic plan view, that is, along the axis z, of a dot 32of circular section. The deposit of magnetic material formed on the sideof this dot, at oblique incidence, parallel to the plane (x, z), hasreference numeral 34.

An information storage medium according to the present invention couldthus comprise an array of dots of the type of dot 32, formed on the samesubstrate, the plane of the array being the plane (x, y).

9) If the easy axis of magnetic anisotropy is perpendicular to the planeof the array of dots, the equivalent of a material with perpendicularmagnetic anisotropy is obtained, since the magnetic material with planaranisotropy (which is the case of iron, cobalt, nickel or their alloys inthin layers) has been deposited on the side of the dots, the side whichis perpendicular to the plane of the array.

The invention offers various advantages relative to conventionalmaterials with perpendicular anisotropy, for example ordered alloys suchas CoPt and FePt or multi-layers such as Co/Pt and Fe/Pt:

it is very easy to form the lateral deposit of magnetic material (infact a single layer of magnetic material), contrary to producing anordered alloy, epitaxied on a monocrystalline substrate, or a multilayercomprising layers whereof the thickness is a fraction of a nanometre. Inaddition, all the magnetic materials with planar magnetisation currentlyemployed for production of information storage media can be used.

This deposit of magnetic material has considerable thermal stabilitycompared to that of multilayers composed of very thin layers of cobalt,nickel, iron, palladium and/or platinum: there is no risk of theelements interdiffusing, leading to the loss of magnetic properties.This considerable thermal stability renders the structure of the mediumforming the object of the invention compatible with existingtechnological processes and also enables thermal treatment under amagnetic field at high temperature to be envisaged so as to induce anaxis of magnetic anisotropy in the deposits formed on the dots of amedium according to the present invention, or to modify the magneticproperties of these deposits.

The total thickness of the material deposited on an dot can be muchgreater than in the case of the multilayers with perpendicularmagnetisation.

The magnetic signal generated by the lateral magnetic deposit is moreintense than that generated by an alloy or by a multilayer of totalequivalent thickness since, in the present invention, it is possible touse magnetic alloys having strong magnetisation, which do not presentperpendicular magnetic anisotropy.

This magnetic signal depends much less on the size of the dots: for adeposit formed on the horizontal surface of a square dot, reduction by afactor of 2 of the lateral size of the dot leads to a reduction by afactor of 4 of the magnetic volume, whereas, for a deposit formed on theside of this dot, this reduction factor is equal only to 2.

This magnetic signal is much more localised on the array of dots: for amagnetic deposit of 10 nm in thickness formed on the side of a dot ofsquare transverse section, of 200 nm sides, the horizontal surfaceoccupied by this deposit is twenty times weaker than that of the samedeposit, formed on top of the dot. This allows possible magneticinteractions, with constant information density relative to a deposit onthe top of the dots, to be reduced, or inversely allows the informationdensity for equivalent magnetic interactions to be increased.

Purely indicatively and in no way limiting, consider a dot of height h(in nm) of square section, of side L (in nm). It is supposed thatmagnetisation of the lateral magnetic layer of this dot is directedalong the axis z and this magnetic layer is considered as being amagnetic dipole.

In the case of a magnetic deposit of thickness t (in nm) formed on theside of the dot, the magnetic volume is L×h×t nm³, for a horizontalsurface of L×t nm², resulting in a magnetic density (magnetisationquantity per unit of surface) equal to h (in arbitrary units).

If this lateral deposit is compared with a conventional deposit of t nmof a multilayer (Co_(0.5 nm)/Pt_(1.5 nm))_(t/2) formed on the top of thesame dot, the magnetic volume is of the order of 0.5×t/2×L×L nm³, for ahorizontal surface of L×L nm², resulting in a magnetic density of t/4.The resulting gain is therefore equal to 4h/t.

The signal gain is proportional to the quantity of magnetic material andis therefore 4 in this example where the two deposits being consideredhave the same thickness, but where the multilayer contains four timesless cobalt than the single layer.

In summary, the present invention makes it possible to have a much moreintense magnetic signal and this on a much weaker lateral surface. Thisis all the more relevant since, in the case of a multilayer(Co_(1/4)/Pt_(3/4)), magnetisation of such layers of cobalt, a fractionof a nanometre in thickness, is weaker than that of the bulk cobalt, onthe one hand due to reduction of the magnetic moment generally observedin the ultra-thin layers (there is formation of an interface alloy) and,on the other hand, due to reduction of the temperature of magneticorder.

If the easy axis of magnetic anisotropy is parallel to the plane of thearray of dots, there is no gain in signal quantity, given that the samemagnetic layer is deposited either on the top or on the side of each ofthe dots of the array, and the resulting gain in localisation is of theorder of h/t relative to a conventional material having planaranisotropy.

In a more quantitative manner and with reference to FIG. 4, if we knowthe volume and the magnetisation per unit of volume of the deposit, thevalue of its surface parallel to the plane (x, y) of the array of dotsand the flying height of a reading head 36 which is supposed to bepointlike and which moves vertically to the magnetic deposit, along theaxis y for cases I and II and along the axis y or the axis x for caseIII, then it is possible to calculate the component Hz, along the axisz, of the magnetic field radiated by each dot above the array of dotsfor different geometries of deposit:

(I) deposit, on the top of the dots 2, of a multimagnetic layer 38 10 nmin thickness, with perpendicular magnetisation of intensity M/4,directed along the axis z,

(II) deposit, on the side of the dots 2, of a magnetic layer 4 10 nm inthickness, with planar magnetisation of intensity M, directed along theaxis y,

(III) deposit on the side of the dots 2, of a magnetic layer 4 10 nm inthickness, with planar magnetisation of intensity M, directed along theaxis z.

FIG. 5 makes it possible to compare the variations of Hz (in arbitraryunits) as a function of the distance y (in nm) at the centre of the dot(parallel to the axis y), for a magnetic field radiated at a distance of20 nm above the surface of the dot which is supposed to be cubic, 200 nmin side, in these three cases (I) to (III), the displacement of thereading head 36 being made along the axis y for cases (I) and (II) andalong the axis y or the axis x for the case (III).

The curves I, II, IIIy and IIIx of the FIG. 5 correspond respectively tocase I, to case II, to case III with displacement along the axis y andto case III with displacement along the axis x.

It is observed that, relative to the case (I) of the multilayer withperpendicular magnetisation, the value of Hz is around 3 times strongerin the case (II) of the deposit on the side of the dot withmagnetisation directed along the axis y, and around 7 times stronger inthe case (III) of the deposit on the side of the dot with magnetisationdirected along the axis z.

In addition, if the reading head is displaced along the axis x in case(III), then the spatial extension of the signal is much weaker than inthe case of displacement along the axis y (the width at half maximum ofthe signal is around 5 times weaker), which causes much betterseparation of the signals emanating from the different dots and alsoleads to a decrease in magnetostatic coupling between dots.

After forming the deposit of magnetic material on the side of the dotsof the array, it is possible to form a protective layer, for examplemade of silica, on the entire surface of the array of dots, then toplanarise this surface to eliminate any relief.

It should be noted that, in the present invention, the dots can be inany shape at all; for example, they can have a square, triangular,elliptical or round section.

In addition, each dot can contain a magnetic domain or a plurality ofmagnetic domains. By way of example, with a dot square in section, it ispossible to get four magnetic domains. But these magnetic domains mustbe separated.

In addition, with reference to point 8 mentioned above, it is specifiedthat in the invention another manner of inducing an axis of magneticanisotropy in the plane of the thin layer is to make a deposit of alayer of an antiferromagnetic material above or below the thin layer.This antiferromagnetic layer is for example NiO, FeMn or PtMn. By way ofexample, this layer can be a few nanometres in thickness.

1. An information storage medium, said medium comprising an array ofdots of submicronic dimensions, said array being formed on a substrate,said dots being raised portions above the surface of the substrate, eachdot containing at least one magnetic domain which has a direction ofmagnetization and corresponds to a bit defined by this direction ofmagnetization, said magnetic domain being formed by a layer of at leastone magnetic material which laterally covers at least one of the sidesof said dot and reaches the apex of said dot without extending along thetop of said dot, wherein each said layers of at least one magneticmaterials directly contacts only a single dot, and wherein each of saiddots is separated from the next closest dot by a space on all sides ofeach of said dots.
 2. The medium as claimed in claim 1, wherein thedirection of magnetisation is perpendicular to the plane of the array ofdots.
 3. A storage medium comprising: a substrate; a plurality of dots,said dots being raised portions above the surface of said substrate,comprising a silicon based material, wherein each of the plurality ofdots has submicronic dimensions and is substantially in a cubical shape;and a layer of magnetic material configured to laterally cover at leasta portion of one side of said dot, wherein each of the plurality of dotscovered with the layer of magnetic material is configured to storeinformation, wherein each dot contains at least one magnetic domainwhich has a direction of magnetization and corresponds to a bit definedby the direction of magnetization, said magnetic domain being formed bysaid layer of magnetic material, wherein said layer of magnetic materialreaches the apex of said dot without extending along the top of saiddot, wherein each said layers of at least one magnetic materialsdirectly contacts only a single dot, and wherein each of said dots isseparated from the next closest dot by a space on all sides of each ofsaid dots.
 4. The storage medium as claimed in claim 3, wherein each ofthe plurality of the dot has a dimension from 10 nanometer (“nm”) to 500nanometer (“nm”).
 5. The storage medium as claimed in claim 3, whereinthe plurality of dots is arranged in an array form.
 6. The storagemedium as claimed in claim 5, wherein the thin layer of magneticmaterial includes iron, nickel, cobalt, or alloy.
 7. The storage mediumas claimed in claim 5, wherein the thin layer of magnetic materialincludes chromium, tantalum, platinum, molybdenum or terbium.